Graphite fiber treatment

ABSTRACT

High modulus graphite fiber reinforced composites with improved interlaminar shear strength are provided by depositing a metal containing compound on the graphite fiber, decomposing the metal containing compound at elevated temperatures in an inert atmosphere and then using the fiber to form a composite.

Elnited States Patent [191- Elban et al.

[4 1 Sept. 3, 1974 GRAPHITE FIBER TREATMENT [75] Inventors: Wayne L. Elban, Westminster, Md.;

James V. Larsen, Salt Lake City, Utah [73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

[22] Filed: Mar. 27, 1972 [21] Appl. No.: 238,262

[52] US. Cl. 117/46 CA, 29/1822, 75/DIG. l, 75/212,117/118,117/160 R, l17/DIG. 11

[51] Int. Cl. C23c 3/04 [58] Field of Search.... 117/46 CA, 160 R, DIG. 11, 117/71 R, 100 B, 162, 118, 107.2 R, 106 C;

75/DIG. 1, 212; 29/1825 [56] References Cited UNITED STATES PATENTS 3,356,525 12/1967 Gutzeit 117/119 3,627,570 12/1971 Cass 117/DIG. 11

Primary Examiner-Charles E. Van Horn Assistant Examiner-Michael W. Ball Attorney, Agent, or Firm-R. S. Sciascia; J. A. Cooke; M. G. Berger [5 7] ABSTRACT 14 Claims, 2 Drawing Figures GRAPHITE FIBER TREATMENT BACKGROUND OF THE INVENTION Carbon fibers are one of the most promising new materials of recent years. Currently there is considerable interest in these high-modulus, highstrength filaments in developing technology for their use in reinforced plastics composites. The attractiveness of carbon fiber composites arises from the fact that the specific modulus and specific tensile strength are very high compared to conventional engineering materials such as fiberglass composites, titanium, steel and aluminum.

At the present time the relatively high price of the fi- Y bers (one of several hundred dollars per pound) has limited their use to applications where weight saving is at a premium. Price projections, however, for largescale production of the fibers have been as low as five dollars per pound. At lower prices the fibers could profitably be used in many conventional applications such as sports equipment, cables, commercial buildings, bridges, and even automobiles.

Other properties of carbon fibers which are of particular interest are their thermal and chemical stability, electrical conductivity, low coefficients of friction and thermal expansion, high strength retention in tensile cyclic fatigue, and resistance to moisture. Carbon fibers offer one other distinct advantage. They are available with a range of mechanical strengths so that fibers still remain. The most significant of these has been associated with the interface area and the development of strong fiber-resin bonds. The weak bond achieved between the matrix resin and the carbon fibers has been the subject of many investigations but still the role of the many interacting factors is not fully understood and is the subject of much debate. Typically interlaminar shear strengths, which are a measure of the fiberresin bond, in untreated carbon fiber-epoxy resin composites are around 3,500 psi. This compares with values of over 15,000 psi for other reinforcing fibers (glass and boron). The general approach to the shear strength problem with carbon fibers has been through fiber surface treatments.

To date the surface treatment approach most widely used has been oxidation of the fiber surface by a variety of processes. Thus oxidation of graphite fibers with air,

'I-INO and NaOH have been used to give improved fiber-resin bonding. Such treatments have produced various desirable and undesirable results. Most of the oxidation treatments require long exposures to the oxidative environment which, on occasion, cause reductions in the fiber tensile strengths along with the increased composite shear strengths. Furthermore in many cases crease in shear strength with no reduction in tensile strength.

Graphite fibers present a more difficult problem than other carbon fibers because graphite is more crystalline than other carbon fibers and it is more difficult to treat such fibers in a manner so as to increase shear strength of composites produced from these fibers. Additionally when one has a relatively high modulus fiber (i.e., one with a modulus of about 45,000,000 psi or more) it is more crystalline than low modulus fibers so that high modulus graphite fibers present 'a particularly difficult problem because not only is graphite more crystalline than'other carbon fibers but the high modulus graphite fibers are more crystalline than the lower modulus graphite fibers.

Thus research has been conducted in an attempt to find a method of improving the shear strength of composites made with high modulus graphite fibers.

SUMMARY OF THE INVENTION Accordingly one object of this invention is to provide graphite fibers.

method for forming graphite fibers which can be used to form composites. I

A further object of this invention is to provide a relatively fast method'of forming high strength graphite fibers which can be used to form composites.

A still further object of this invention is to provide a method for forming high modulus graphite fibers which can be relatively strongly bonded to other materials to form composites.

- highmodulus graphite fiber a chemical compound growing of silicon carbide whiskers on the fibers but it is desirable to have alternate methods to achieve an inwhich contains either Fe, Co, Cr or mixtures thereof (hereinafter called a metal containing compound) and then heating the fiber with the material deposited thereon to a temperature above the decomposition point of the metal containing compound in an inert atmosphere to form the desired treated fiber. This treated fiber is then used to form composites in conventional manners.

BRIEF DESCRIPTION OF THE DRAWINGS Still other objects and many of the attendant advanta'ges of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which:

FIG. 1 is a diagram of the apparatus used to apply the metal containing compound onto the fiber and then dry the solvent; and

FIG. 2 is a diagram of the apparatus used to decompose the metal containing compound.

DESCRIPTION OF THE PREFERRED EMBODIMENT -20. When the fiber'emerges from the treatment solution it is guided around pulley 22 to drying tube 24. Hot air gun 26 is positioned so that it blows hot air into the drying tube so that the solvent if any is evaporated. Va-

riac 27 is used to heat tube 24 to aid in drying the solvent. The fiber is then guided around pulley 28 and is collected on take up spool 30.

FIG. 2 is a diagram of the apparatus used to decompose the coated fiber wherein the fiber 32 obtained from the apparatus of FIG. 1 is unwound from spool 34, passes over pulley 36 and enters heating chamber 38 by passing over graphite pulley 40. The fiber exits the chamber by passing over pulley 42. The fiber is heated while in the chamber by resistance heating. Thus the electrical circuit includes the portion of the fiber that is between the graphite pulleys, electrical wire 44, variac 46, electrical wire 48, ammeter 50 and additional electrical wire 52. The temperature of thefiber is obtained by using an optical pyrometer. Upon passing out of the chamber the fiber passes over pulleys 54 and 56 to take up spool 58. Gas inlet 60 is positioned so as to allow the introduction of gas into the entire decomposition system since the decomposition step of this invention is to be conducted in an inert atmosphere. Gas outlet 62 is also provided for the egrees of gas that is in the system.

These fibers are dipped into the treatment solution which is preferably in the form of a dilute solution. It is preferable to apply the'metal containing compound to the fiber from dilute solution because a relatively small amount of pickup is desired. Thus, after the solvent has dried it is desirable to have the metal contain-' ing compound constitute. 0.10 to 3 weight percent of the total fiber plus metal containing compound weight with 0.25-2 weight percent being most preferred. Thus the concentrations of the solutions are relatively low and are under 10 percent by weight with 1-5 weight percent being most preferred. However, although concentration is not critical better results are obtained if the preferred concentrations are used.

Since the metal containing compound is preferably applied from solution one requirement of the metal containing compounds used in this manner is that they be soluble in some solvent. Furthermore, since the solvent is to be evaporated it is desirable that the solvent be low boiling. Although any metal containing compound or mixtures thereof that are soluble in a solvent which does not attack the fiber can be used there are certain compounds which are preferred. Thus metal containing compounds such as ferric chloride, dicyclopentadienyliron (ferrocene), ammonium ferrocyanide, ferric oxalate, ferric citrate, ferrous ammonium sulphate, chromium acetylacetonate and dicyclopentadienylcobalt have been used in this invention. It should be noted that when the metal containing compound is an organic compound the temperature of the heating step converts the organic compound to carbon char so that a final product will be obtained which comprises a graphite fiber with carbon char and the metal thereon. To form carbon char the heating temperature must be at least equal to the decomposition temperature of the organo-metallic compound. Formation of carbon char from compounds is more fully discussed in Ser. No. 238,261 entitled Carbon Fiber Treatment by Joseph M. Augl, James V. Duffy and James V. Larsen filed on the same date herewith and hereby incorporated by reference.

The decomposition step is conducted in the apparatus of FIG. 2.'The graphite fiber with the metal containing compound or mixture of compounds deposited thereon is heated to the decomposition temperature of the metal containing compound in an inert atmosphere. In the apparatus depicted in FIG. 2 an inert gas is introduced into the heating chamber. The heating of the fiber is affected by passing an electric current through the fiber by contacting the fiber with two graphite electrodesThe fiber are heated to a temperature at least equal to the decomposition temperature of the metal containing compound in order to obtain the proper interaction between the metal and the fiber. The upper limit for heating is the temperature at which the fiber itself decomposes or the temperature at which the fiber, any metal on the fiber orcarbon char if any is present react with the inert gases in the heating chamber. Thus within the context of this invention the term decomposition temperature of the graphite fiber is meant to include not only the temperature at which the graphite fiber decomposes but also the temperature at which the graphite fiber, or the metal that is deposited on the graphite fiber or carbon char that is on the fiber (if any) react with the inert gas present in the chamber.

The graphite fibers thus formed have deposits of a metal thereon. The metal can be Fe, Co, Cr or mixtures thereof. If a metallo-organic was used to coat the fiber carbon char is also present.

The graphite fibers thus obtained can then be used to form composites. These fibers are to be used in the same manner as other graphite fibers in the prior art. However, the products obtained using these fibers have better interlaminar shear strengths, due to better fiberresin bonding, than do the prior art graphite fibers.

It should be noted that within the meaning of this invention the term metal containing compound is defined as any compound which contains a metal selected from the group consisting of Fe, Co and Cr and mixtures of compounds which. contain any one or combination of these metals.

The general nature of the invention having been set forth, the following example is presented as a specific illustration thereof. It will be understood that the invention is not limited to this specific example but is susceptible to various modifications that will be recognized by one of ordinary skill in the art.

EXAMPLE 1 Ferrocene is dissolved in toluene to yield about a 2 percent by weight solution. The untreated graphite fiber is moved through the dilute solution at a travel rate of 10.5 fpm so that the residence time of the fiber in the solution was about 4 seconds. The solvent was dried in the drying tube. The dried fiber was then put through the heating tube of FIG. 2. The fiber residence time in the heater was about 6 seconds at a temperature of about 800C.

The fibers thus obtained were fabricated into composites using the standard techniques. Thus the fiber obtained above was combined with ERLB 4617 (Union Carbide Corp.) a cycloaliphatic epoxy resin and a curing agent, methylene dianilene. The ratio of resin to curing agent was 100:46 parts by weight. The composite was cured at 85 C for 4 hours, at 220 C for 3 hours and at 150 C for 16 hours.

EXAMPLE 2 The same procedure as was used in Example 1 was repeated except a 3 percent by weight solution of FeCl;, in benzene, and another one in water, was used. Again composites formed from the treated fibers were superior to untreated fibers with respect to shear strength.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A method of treating a graphite fiber comprising:

depositing on a graphite fiber a organo-metallic compound wherein said metal is selected from the group consisting of Fe, Co, Cr and mixtures thereof; and

heating the graphite fiber with said organo-metallic compound deposited thereon to a temperature in excess of the decomposition temperature of said organo-metallic compound but below the decomposition temperature of the fiber.

2. The fiber obtained by the process of claim 1.

3. In composites comprising graphite fibers the improvement comprising incorporating the graphite fiber of claim 2.

4. The method of claim 1 wherein said graphite fiber is a high modulus fiber.

5. The method of claim 4 wherein said organometallic compound is deposited on said high modulus graphite fiber from a solution and the solvent is evaporated prior to decomposition.

6. The method of claim 5 wherein said organometallic compound is selected from the group consisting of dicyclopentadienyliron, ferric oxalate, ferric citrate, chromium acetylacetonate, dicyclopentadienylcobalt and mixtures thereof.

7. The fiber obtained by the process of claim 4.

8. In composites comprising graphite fibers the improvement comprising incorporating the graphite fiber of claim 7.

9. The fiber obtained by the process of claim 6.

10. In composites comprising graphite fibers the improvement comprising incorporating the fiber of claim 11. The method of claim 1 wherein said organometallic compound is deposited on said graphite fiber from a solution and the solvent is evaporated prior to decomposition.

12. The method of claim 11 wherein said organometallic compound is selected from the group consisting of dicyclopentadienyliron, ferric oxalate, ferric citrate, chromium acetylacetonate, dicyclopentadienylcobalt and mixtures thereof.

13. The fiber obtained by the process of claim I2.

14. In composites comprising graphite fibers the improvement comprising incorporating the fiber of claim 13. 

2. The fiber obtained by the process of claim
 1. 3. In composites comprising graphite fibers the improvement comprising incorporating the graphite fiber of claim
 2. 4. The method of claim 1 wherein said graphite fiber is a high modulus fiber.
 5. The method of claim 4 wherein said organo-metallic compound is deposited on said high modulus graphite fiber from a solution and the solvent is evaporated prior to decomposition.
 6. The method of claim 5 wherein said organo-metallic compound is selected from the group consisting of dicyclopentadienyliron, ferric oxalate, ferric citrate, chromium acetylacetonate, dicyclopentadienylcobalt and mixtures thereof.
 7. The fiber obtained by the process of claim
 4. 8. In composites comprising graphite fibers the improvement comprising incorporating the graphite fiber of claim
 7. 9. The fiber obtained by the process of claim
 6. 10. In composites comprising graphite fibers the improvement comprising incorporating the fiber of claim
 9. 11. The method of claim 1 wherein said organo-metallic compound is deposited on said graphite fiber from a solution and the solvent is evaporated prior to decomposition.
 12. The method of claim 11 wherein said organo-metallic compound is selected from the group consisting of dicyclopentadienyliron, ferric oxalate, ferric citrate, chromium acetylacetonate, dicyclopentadienylcobalt and mixtures thereof.
 13. The fiber obtained by the process of claim
 12. 14. In composites comprising graphite fibers the improvement comprising incorporating the fiber of claim
 13. 